fpso conversion

8/20/2019 FPSO Conversion

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Guillaume Gourdet - Paulo Biasotto - !"!#

I. Abstract

FPSO conversion projects experimented a sharply increasing since 1996, many ofthem converted from single hull tankers built before 1985. First inspections haveshown a number of failures of structural components indicating that hull integrity ofthese units will be a challenge as they should stay on station along the field lifewithout go on dry-dock. Thus a rational inspection plan and maintenance need beset up in order to avoid or minimize the impact of inspections in the normal operationof the unit. Importance of the conversion is also outlined through return of experienceon converted and guideline in the choice of the hull and the preparation ofconversion works.

II. Introduction

Although new built F(P)SOs have been significantly increased during the last years,in particular for use in harsh environment zones and deepwater developments wherehuge are required, converted s are still a commonly chosen solution for milderenvironment like West Africa, South East Asia and Brazil. At the moment, with fully-booked shipyards, they are the best solution for fast tracks projects where earlier firstoil is required.Today there are approximately 111 FPSOs and 86 FSOs systems [10] in operationor available worldwide, approximately 70% are converted from trading single hull

tankers built along the 70s and early 80s. Many of these units have been inoperation already after 20-30 years and shall stay on site during at least more 10-15years, where disconnection or removal from the moorings systems is not planned.That gives on one hand a good return of experience on the structure of that type ofunit, but on the other hand a great challenge to maintain as long as reasonablypossible these units on site.The low price of single hull tankers is the main reason why conversion is still mainlya single hull point, despite the aging of single hull ships and their scarcity. The paperwill discuss the types of single hull vessels, summarizing their main characteristicsand the common way of assess the hull structure strength of these vessels. It willthen focus on the possible repairs based on return of experience, and will end on theway to avoid costly repairs: a worthwhile conversion, efficient inspections andrational maintenance plan.


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III. Single hull vessels

In service single hull vessels have generally been built between the mid 1970’s and1995. After 1996 double hull tankers building have known a sharp increase, puttingforward Japanese and Korean shipyards.

The selection of single hull tankers for a conversion project is leaded by the balancebetween the purchasing cost of the hull, lower for single hull ships than for morerecent double hull ones and the refurbishments work, which depends on the age andprevious maintenance of the ship.The table below shows the price of tankers depending of their age. It also shows aseparation in single hull tanker around 1985-86. After 1996 most of the vessels aredouble hull tankers.

Oil TankersAverage Sale Price (m USD)

Year of building 1973-

1985

1986-

1995

1996-

2000

2001-

20042000 11.8 43.0 73.8 -2001 12.6 35.2 79.2 82.52002 11.0 21.5 59.5 75.0

2003 13.0 32.8 52.0 65.42004 15.0 60.0 85.5 110

Average sale price 12.3 36.3 66.1 88.3Ships operating 29 185 116 118

Table 1 - oil tanker average sale price (source: CW Kellock $ % &'

III.1/ Vessels built before 1985

Conversion of single hull tankers built before 1985 have been a worthwhilealternative for FSO projects. Such vessels are relatively cheaper and their primaryand secondary structures use to be stiffer than in hulls built after 1985, where hightensile steel has been extensively used.A number of different designs can be identified, comprising American, French,German, Swedish and Japanese standards among others. The industry has a goodreturn of experience with oil tankers and an extensive list of areas prone to defectsdue to stress concentration and corrosion is given [1], [2], [3] and [9]. Nevertheless, itis well known that occurrence of corrosion and other defects might vary according tothe shipyard design, fabrication standards, workmanship and the operation andmaintenance procedures adopted during the trade period.There are a number of differences and particularities inherent to each shipyarddesign, but one of the most important characteristics is the primary structurearrangement. Generally two main types of structural arrangement are noticed:

o Longitudinal ring stiffener system comprising deep girders within centerand side tanks, supporting the transverse bulkheads horizontally stiffened.

There are no horizontal stringers.o Horizontal system comprising in general four stringers within center and


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side tanks, supporting the transverse bulkheads vertically stiffened. Ingeneral a longitudinal ring is provided in way of the center line girder.

Fatigue was clearly not the main concern in the design of connections of primary andsecondary elements for such type of vessels, but in some way, it was apparentlycompensated for the stiffer structural arrangement, larger corrosion margins and less

use of HTS.During this period, oil tankers were built worldwide in different shipyards, some ofthem having poor fabrication quality.

III.2/ Vessels built after 1985

On the contrary of vessel built before 1985, the structure design of ships built after1985 is optimized by means of finite element calculations and the extensive use ofHTS to reduce the weight of steel. The hull structure design has used ST355, ST315and ST235 steel types in different manners, in particular in the side shell plates,stiffeners and web frames.Some of them are built only using HTS in the cargo area. The use of HTS, inparticular in the side shell area around the neutral axis, was found to be one of themain causes of problems associated to this type of vessel. Fatigue cracks of sideshell longitudinal connections to transverse primary structures are probably the mosttypical defect of such designs, as fatigue strength of side shell longitudinals andcorrosion margins are affected by the less stiff structural panels at bottom and sideshell areas.As a general rule, hulls built with ST355 should be specially regarded as suchvessels have an extensive record of defects and likely have been reinforced withadditional brackets after construction and before the second Class Renewal Survey.

Another important parameter is the use of asymmetric profiles, i.e. angle stiffeners inthe side shell and bottom panels. The use of angles at those locations having highprobability of failure should be specially considered when evaluating hulls candidateto conversion, in particular when associated with use of steel ST315 and ST355.

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Table 2 - Material on typical midship sections of single hull units


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IV. FPSO Hull Condition AssessmentThere are approximately 197 s systems in operation or available worldwide,approximately 60% are converted from trading single hull tankers. These units havea service of already more than 20 years, where the two major threats for corrosionwastage and fatigue defects. Threats, which can be of different intensity depending

of the units design, use and maintenance.The following studies are in general the most common ways to assess the hullgeneral condition:

Structural analysis will determine the weak locations of the structure,taking into account the state of the vessel, and its future operations andsite conditions.

o Yielding and buckling analysis of the “as-is” state will allowdetermine minimum of steel renewal and reinforcement needed atconversion phase. Generally older ships have quite a margin onyielding and buckling criteria. However important corrosion canreduce significantly this margin, in particular on horizontal surfaces(web of side longitudinal stiffeners, horizontal stringers, bottom…).

o Corroded FEM model. On a ship corrosion is everywhere. Onetough question of repair work is to estimate which of the corrodedelement should be renewed and which should not. It is quiteobvious when corrosion is above class society criteria. But whenthe corrosion is just below those criteria, the owner will decide if theplate/stiffeners should be kept - then the risk to have to change theplate because of excessive corrosion in the following years isimportant - or if the plate/stiffeners should be renewed atconversion - which guarantees to avoid to renew the plate for

several years but is most costly. To fix the limit between renewaland not, it is common to perform structural assessment of the unittaking into account corrosion estimation at several stages of the unitservice life – generally every 5 years. These calculations will givecorrosion criteria for each element, depending of the loading of thestructure. However these criteria are based on corrosion estimation,which are always quite critical since corrosion is a randomnessphenomena. So it should not discharge of a good maintenanceplan!

200X 200X + 5 years 200X + 10 yearsTable 3 – Corroded finite elements models of aft centre tank and wing tank. Red

elements show structural areas with don’t comply with classification rules in 2004,2009 and 2014. The scantling of these 2 last years have been estimated based on

several previous UTM report for this unit.


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Surveys will show if the ship is in good state from corrosion point of view. It

will also reveal details and areas which are or had cracked or buckled.These areas should particularly be checked in the structural analysis, andadequate repairs should be put in place. The survey should also review

the hot spot found during the structural analysis.o Coating survey. A good ‘hard’ coating is a way to slow – if not to

stop – corrosion. It can be efficiently used when structuralassessment shows that corrosion margins are low. Generallyballast tanks (tanks which are used as ballast on the F(P)SOs butalso which have been used as ballast during trading life) are fullycoated. Cargo tanks are often coated on the lower and upper part.However any coating breakdown will allow corrosion process totake place and generally at a much more accelerated rate. So theeffectiveness of coating has to be monitored throughout the unitservice life by CVI and UTM (see section V.2.1/) in case of

suspicion of breakdown.o Wastage and feet of anodes should be surveyed, for replacement if

necessary.o Special care should be taken to the hull in way of the location where

supply boats berth. If not protected, this area is a well know area ofhigh risk of deformations due to collision.

Study of operation and maintenance of the trading ship. Old ships mighthave cargo tanks used for heavy ballast in the past. Those tanks are proneto excessive corrosion and should be especially checked. In maintenancereports, repetitive repairs of same type of details revealed that the detailsare not correctly design. A careful review of ship inspection and repairreports are necessary to detect this type of defects and to know at whichfrequency it generally breaks down. Structural analysis taking into accountthe site environment conditions will help defining the minimum necessaryreinforcement

V. Maintenance

Deteriorations modes such as corrosion and fatigue are initiated during the tanker

trading phase, more or less mitigated by conversion works and continued during allthe F(P)SOs service life. Yet during the service life, dry-docking the ship for repair isgenerally not planned. The cost of dry-docking and of the unavailability of the unitsduring the several weeks of route to dry-dock is prohibitive. That’s why when it ispossible repairs are done on site, with minimum impact on production. These typesof repair have their own constraints as shown in the following sections.


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V.1/ Repair works on site

V.1.1/ Types of repair

The main types of repairs mentioned in this paper are

o Coatingo Anodes replacemento Re-welding of cracked weldso Local inserts (for pitting and fatigue crack repairs)o Renewal and/or fitting of structural elements (in case of global corrosion,

or of reinforcement)Temporary repairs, like crack arrestors holes, which are commonly used in shippingindustry, should be considered with particular care in case of offshore facilities. Theaim of crack arrestor holes is to stop the crack for a few months waiting for the nextdry-dock. This is rarely the case for a station keeping unit.If coating and anodes replacement can be considered as maintenance repairs, the

three last ones almost always imply exceptional repair plan. They also imply hotworks, which on an offshore unit is not a minor point.

V.1.2/ Hot works on a operating F(P)SO

Hot work on an F(P)SOs raises a lot of issues concerning the security of the peopleonboard and of the unit. The oil and particularly the vapor of the oil are highlyinflammable. Available Guides or Rules are not adequate to repair of floating storageunits on station. To cover this lack some companies follow the ISGOTT (InternationalSafety Guide for Oil Tankers and Terminals), which applies to trading oil tanker andonshore terminals. The main issue about applying the ISGOTT is to condemn the

tanks adjacent to the one within repair have to done. These tanks will have to be gasfreed or inerted to avoid any ignition hazard.The problem when closing several adjacent tanks is to know if the unit can beoperated with these tanks empty. The following points should be investigated beforeany decision:

o Reduction of storage capacityo Hull girder strength question. If the tank to be repaired is in midship area,

maintaining empty adjacent tanks will create high hull girder bendingmoment and shear. These hull girder loads have to be assessed andchecked below permissible values.

o Operating capacity of the unit if particular tanks are condemned (slop

tanks, process tanks)


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General Arrangement of typical single hull tanker (Length about 310m)4 .

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Case with hot works in center tank 3

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Case with hot works in wing tank 3 (ballast) 8 . 5

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Case with hot works in aft centre tank

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Table 4 – Reduction of oil storage due to strict application of hot works rules.

All these questions have to be answered on a case by case basis, depending on thegeneral arrangement of the ship, tanks to remain empty and operating conditions ofthe unit.

Due to the difficulty to answer to all these questions, some operators have chosen tostop the unit during the repair work on site. In such case, depending on the fieldoperation, shutdown may be avoided either by by-passing the oil directly to a shuttletanker – the F(P)SO being used only for the process and pump room – or by leasinga station keeping floating unit for the duration of the repairs. Even in such caserepairs might be done on site to avoid to disconnect, to unmoor and to tow the unit toan available dry-dock.

Instead of applying ISGOTT recommendations, some companies defined their own

on-site repair guides. These guides are generally based on the location of hot worksin tanks to define if adjacent tanks should be kept empty or not. They allow repairing


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far from the bulkheads at borders of tank without closing the adjacent tank. Thislimits the number of tanks unavailable during repair, but does not always solve all theissues listed above.

V.1.3/ Feasibility of repairs on site

Another question when doing repairs on site is the feasibility of these repairs andtheir quality. Some repairs are not or hardly feasible afloat, such as extensive repairof side shell, deck or bottom areas. There are some techniques using watertightboxes to weld inserts on a boundary with sea (bottom, side shell), however theyrequire cutting of longitudinal plating and stiffeners contributing to the hull girder.Such discontinuities in the hull girder have to be carefully analyzed. The repairquality will also depend on the quality of alignment of the renewed elements. In suchconditions – afloat with continuous incoming waves, in a confined space - respectingalignment tolerances is not an easy challenge, but is a critical point for elements

contributing to the hull girder.This is an extreme example of difficulty to repair on site, where a simple work in dry-dock can become a tough task afloat at all stages of the repair process. Fromcleaning process, which highly depend on water pump power and on capacity totreat dirty water up to the management of people onboard, where places are limitedand security a first priority.

Preparation of a tank for repairs means cleaning, eventually removal of all thesediment and mud in way of repair areas, sufficient ventilation and lighting for thenumber of workers. In some countries, it can turn out quite difficult to remove thesediments of the unit, as disposal might not be accepted onshore.

Another point is the work in confined space (see ref [5]), as ventilation, lighting,security exit will limit the number of people working at same time within the tanks. Ifhuge teams of workers have been seen working almost 24h on some units, itrequires a difficult and well done health and safety management plan.

The following paragraphs list the typical repair works cited in section V.1.1/, and foreach mention problems that can be encounter in case of afloat repairs:

The quality of coating is essential. A bad coating might lead to quick breakdown andconsequently to inefficient corrosion protection. Such areas may have anaccelerated wastage. Three points are directly linked with the quality of coating:

Qualification of worker and of the coat used. This point is independent ofafloat or not repairs.

Preparation of the surface. This point can be critical since to prepare thesurface takes time, and time is not always available during repair plan,moreover in case of on site repairs.

Atmosphere control. Humidity and temperature need to be controlled andmaintained in acceptable values for the coating to correctly dry. If this isalready difficult tasks in shipyards, it is even more difficult on board, withlimited ventilation capacities, offshore natural humidity and oil and watertemperature in adjacent tanks.

Changing anodes, by cold cutting and bolting or welding doesn’t raise particular

problems. Special care should be taken with bolted anodes which are prone to loosecontact by corrosion with the hull and thus become inefficient.


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Fitting local insert for corrosion like pitting or cracked details raises the samequestion than the example of bottom plating renewal: possibility to cut the damageelement, tolerance of alignment. The loading conditions chosen for the repair period,time should lower as much as possible deformations and stresses in way of thedamaged detail. Else remove it, could become dangerous task and welding the

insert in a highly stressed loading conditions can induced important residualstresses. In case of bad weather forecast after removal of the damaged detail, itshould also always be possible to fit the new insert before harsh weather arrives.To renew whole parts of structure increases the alignment problems. In fact it is quitedifficult to manipulate big pieces of steel inside the tank. This handling limitation isthe main reason of why most of the repairs are done by several little inserts ratherthan by renewed once big piece of structure.

Finally the only way to avoid extensive and costly repair works is to have a fullconversion, taking into account the expected life of the site, and to follow an efficientmaintenance/inspection plan. This will be described in the two following sections.

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Table 5 – Example of typical defects and their common repair solution


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V.2/ Integrity management

V.2.1/ Inspection

To avoid costly and unexpected repairs during field life, a maintenance plan has to

be put in place at the conversion stage. This maintenance plan should include arational inspection plan, answering to the three main questions of inspections: when,what and how.

When inspect?When inspect is really the first question to be answered. When compared withothers offshore structure (like jacket or semi-submersible), F(P)SO hull structureshave different function. The structure of a F(P)SO should support topside andaccommodations (like the others), but also store products (oil, gas). This means thatduring inspections, one or several storage tanks will be unavailable. Theunavailability of tanks is longer that the time of inspection, since the cargo tanks

should be prepared for the inspection. The procedures to enter a tank (cleaning, gasfreeing, stripping and ventilation for a tank of several thousand of cubic meters) takeseveral days, depending on the cleaning and gas-freeing capacity of the unit. Thusthe main constraint of inspection of F(P)SO structure is to avoid affecting theproduction and particularly the off-take schedule.Therefore inspections need to be planned early and to be well prepared, to avoidspending too much time in the tank and reduce the unavailability of the tanks.

The minimum inspection plan for a classed unit is the class society inspections.However with continuous survey notation, the owner can defined when each tanksshould be inspected in the Rules interval. The inspection schedule is generally

decided to minimize the impact of inspections in the operation of the unit. Thisschedule is likely to be modified depending on the results of inspections and on themaintenance and mitigation actions.It also bases on inspection loading conditions defined during the conversion projectphase and approved by the class society.These loading conditions are defined to allow inspection of each tanks taking intoaccount:

the impact on the field operations Trim and stability check Strength assessment of the loading conditions (hull girder loads, local

pressure on bottom, bulkheads)Loading conditions to performed tank testing should also be considered.

On the back of defining when inspection are to be done, inspections plans shouldalso include what should be inspected and how.

What inspect?The scope of a class inspection of a F(P)SO is done on class requirements. Theserequirements depend on the type of tanks, on the age of the unit, on the previousinspections recommendations and on structural analysis reports which can identifypotential critical areas.

The inspections on hull structure don’t only include search for structural defects, butalso for corrosion protection systems: coating and anodes. Generally ballast tanks


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are fully coated and fitted with anodes. On the opposite cargo tanks are often notcoating, or just the upper and lower part. Anodes should be looked at and wastageevaluated. Coating quality should be noted in the survey report. Breakdowns ofcoating often lead to accelerated corrosion in way of the breakdown. Thus theyshould be sought and state of steel below the coating breakdown should be

assessed (by CVI, and if deemed necessary by UTM measurements).

How inspect?Inspection methods are to be selected in accordance with the degradationmechanism expected and its respective type of defect. As said before, 2 mainsdegradation mechanisms affect the hull structure of FPSO:

Cracking: fatigue cracks Corrosion: uniform and localized corrosion, pitting and grooving

A third type of damage can be found on the hull, but due to accident, not the aging ofthe structure:

Mechanical damage: dents, contact and collision damages

The most common methods necessary to control the deterioration process aredescribed here after:

1. GVI (General Visual Inspection) is used for determining the general conditionof the tanks and spaces in order to identify deterioration of any component. Ingeneral, the GVI scope includes coating condition, rust staining and structuraldeformations of plating and associated stiffeners. When defects are suspectedduring a GVI, a CVI is done.

2. CVI (Close Visual Inspection) should be extended as deemed necessary,taking into account the maintenance of the tanks under survey, the condition ofthe corrosion prevention system and also where tanks have structuralarrangements or details which have suffered defects in similar spaces or onsimilar ships according to available information. It requires good vision, goodlighting and the knowledge of what to look for. The surveyor is to be withintouching distance of the weld or component to be examinedCVI is used extensively to evaluate the condition or the quality of a weld orcomponent. It is the primary evaluation method of many quality controlprograms. Therefore, when visual results indicate anomaly or defect, it is oftenenhanced by other methods of inspection which can identify defects that arenot easily seen by the eye.

3. Rafting is a way to access to upper structures of cargo tanks. It consists to fill

the tank with water little by little, while the inspection team is on a raft. It allowsperforming easily CVI or flaw detection on deck transverse or decklongitudinals. However rafting need water pumps to fill the tanks and watertreatment capacities which are not always available on offshore units.

4. CCTV (close-circuit television) associated with rope access is another wayto access to upper structures of cargo tanks. It consists of a team of climbersequipped with a camera and walkie-talkie. The climbers follow the instruction ofthe surveyor, which can survey the details of the upper structures on a TV. Itavoids to fill the tanks with water, or to build scaffoldings.

5. Flaw Detection techniques are used to detect or confirm steel or welddefects, like cracks. Several methods exist, for different type of defects and with

different preparation procedure and probability of detection. But for each ofthem it is critical to have well trained people to follow approved procedures and


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to be able to analyze the results. The following list show the three mainmethods used on hull structure:

a) Eddy Current Inspection (EC)b) Alternating Current Field Measurement (ACFM)c) Magnetic Particle Inspection (MPI)

6. UTM (Ultrasonic Thickness measurement) uses sound waves of shortwavelength and high frequency to detect flaws or measure material thickness.On the back of UTM asked by class societies, suspect areas should be subjectto UTM as considered necessary by the Surveyor, therefore where wastage isevident or suspect it should be carried out. When thickness measurementsindicate substantial corrosion, the number of thickness measurements is to beincreased to determine the extent of substantial corrosion.

The above paragraphs described inspection methods of tanks. It can also be appliedto deck and outside structures. However special inspections have to be carried outfor the underwater structure and the ICCP system protecting the hull from corrosion.

7. IWS (In water survey) is required twice in a five-year period (maximuminterval of 3 years) in lieu of dry-docking. In order to assure a continuingsuitability of the cathodic protection system, it is recommended a conditionmonitoring routine based on such requirements. This requires the use of ROVor of divers (with CCTV system) to confirm the condition of the hull exterior. Itoften links with submarine inspection of mooring systems, risers and offloadinghoses.Diver inspection is required in order to examine the hull and the sea inlets. Seachests should be opened and cleaned and valves checked.It might be necessary to remove marine growth at some locations in order toexamine the hull welds and coating condition. In case of marine growthcleaning is necessary, the work must be carried out carefully in order to avoiddamage of the painting.

These inspections have no impact on the production. However the in-water survey isto be carried out with the ship at a suitable draught in calm water conditions; the in-water visibility is to be good and the hull below the waterline is to be sufficiently cleanto permit proper examination. So the schedule of in water inspection is important andshould take into account draft of the unit and typical weather at the period of the yearwhen it is done.

ICCP system (Impressed Current Cathodic Protection) is an electronic systemprotecting the hull from corrosion by maintaining an electric potential between hulland sea water. The assessment of the efficiency of the ICCP system is performed bymeasuring the steel to sea water potential and inspecting the cathodic potentialequipment. Measurements and inspections are necessary for:

Checking that protection has been achieved according to design criteriaand that each part of the cathodic protection system is operating properly;

Detecting performance changes of the ICCP system because of workingconditions variations with time.

It is to be noted that changes in the ICCP system to maintain protection are

necessary to meet changes with time of conditions which affect protection.


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Table 6 – Example of extension of CVI for a class renewal survey of single hullconverted to fixed unit. (Extracts from the ref [8])

An inspection plan has to include when each of these methods should be used; inorder to have the tools and the team (for flaw detection techniques, or CCTV)available during the survey.

As part of the inspection, tank testing should be performed for tanks which are

intended to contain water or oil. It consists in filling tanks up to the upper boundary ofthe tank, and inspecting boundaries to insure that no leak or deformations appeared.

Age of unit (in years at time of class renewal survey) Locations

age


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Tank testing requires a procedure to be able to inspect each mandatory boundary inthe Rules interval.

Age of the unit in years

age 5 5 < age 10 10 < age 15 age > 15

All ballast tank boundaries All ballast tank boundaries All ballast tank boundaries All ballast tank boundaries

Cargo tank boundariesfacing void spaces, pipetunnels, representative fueloil tanks, pump rooms orcofferdams




All cargo tank bulkheadswhich form the boundariesof segregated cargoes

All remaining cargo tankbulkheads

All remaining cargo tankbulkheads

Table 7 – Example of tank testing requirements from Bureau Veritas Rules

For such a complex project than a whole F(P)SO complete knowledge of inspectionsmethodologies and “how-to” are quite difficult to achieve. That is why on severalprojects document is built which regroup all the information necessary to schedule,follow and archive inspections of the unit, whatever the reasons of the inspection –class requirements for the hull, builder specifications for particular parts of mooringsand piping, company specifications if any.

In order to optimize inspection in term of time spent (i.e. cost of inspection) and ofefficiency of inspection (i.e. probability of detecting default) Risk Based Inspection(RBI) plan can be used. RBI will study the risk linked to each details of the structureand determine when and how it should be inspected to get acceptable low risk. So itwill specify the frequency of inspection of each tank, the details that should beinspected in each tank and the mean of inspection.This plan, if approved by class society, can replace the Rules mandatoryinspections.

V.2.2/ Maintenance

As consequence of inspections, maintenance actions might be required. The aim ofa maintenance action is to maintain the structure in a good enough state to avoidunexpected and costly repairs. Maintenance actions are generally quite short and

light actions that do not need to be planned long time in advance. The mainmaintenance actions are:

Cleaning (mainly for inspections purpose) Retouching of coating where it breakdowns. This point is very important,

since very often corrosion is much quicker when it takes place in coatingbreakdown.

Mitigation procedures or coating of very localized corrosion Replacement of anodes Etc…

The same difficulties are encountered for these maintenance actions than for therepair on site. To limit the unavailability of the tanks, it is possible to keep the tools

(coating for example) to perform minor maintenance on board during the inspection.


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However maintenance of a F(P)SO is not limited to actions on structure. It alsoregroups all kind of actions that insure integrity of the unit in all eventualities. Inmaintenance tasks we can also add such things as:

Keep a traceability of every defects, recommendations, maintenanceactions or repairs made on the unit before and after the conversion. This is

quite a big challenge, since it represents a lot of documents and ofdifferent interveners:

• as-built drawings,

• conversion drawings, inspections reports, shipyard reports

• Class reports, UTM reports, owner reports concerning operations,actions, accidents etc…

When it comes to define inspection or repair plans, this kind of informationare really important and help to focus the work directly at the most criticalareas. It also helps understandings which kind of default is in question(design error, operations error, accident) and thus gives a quick andappropriate answer.

Choose the loading conditions with marine knowledge (avoid alternateloading conditions to limit hull girder loads, let the ballast tanks filled aslong as possible for the anodes to be as efficient as possible).

Keep records of eventual collision (with supply boat for example)

New tools are now being developed and implemented to help owner maintaining theunit. Generally they are Internet-based softwares which allow authorized people toaccess to whole or part of information everywhere around the world. Access andupdate of the database is then possible from head office of the company, from localoffices or from board if Internet is available.

Figure 2 – Examples of screens of web tools storing and organizing inspection,maintenance and repairs history of a FPSO.


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They store all kind of information, from as-built drawings, to pictures of the latestinspection, including classification reports and certificates. They are organizedspecifically for each unit, with access to each tank and areas of the unit informationby clicking on the unit general arrangement.

VI. Conclusion

Converted single hull units are now a great challenge regarding inspection,maintenance and repairs. The last years have revealed how repairs on site aredifficult and costly; but they have also helped defining guidance and practice foroffshore repairs, despite the difficulty to conceal hot works on oil storage unit,security, and operations.Maintenance and inspection of units are known as the best way to avoid unexpectedrepairs after the conversion. They are being optimized in term of efficiency and ofreducing impact on operations through documents which regroup all the rules andguidance requirements and through RBI inspection plans.Web tools are also developed to store, class and share to all the concerned peopleall kind of information concerning the unit (maintenance actions, inspection plans,drawings, pictures), and this worldwide and instantaneously.Finally the key of the success of the challenge of maintenance and inspection is inthe preparation of inspection and maintenance plan based on strong and sharedknowledge and understanding of the unit and of her history.

VII. AcknowledgementThe authors wish to thank Bureau Veritas for permission to publish this paper. The

views expressed are those of the authors and do not necessarily reflect those ofBureau Veritas.

VIII. References

[1] Guidance Manual for the Inspection and Condition Assessment of TankerStructures – Tanker Structure Cooperative Forum, Witherby & Co. Ltd., 1986[2] Condition Evaluation and Maintenance of Tanker Structures - Tanker StructureCo-operative Forum, Witherby& Co. Ltd., 1992.[3] Guidance Manual for the Inspection and Condition Assessment of Double HullTanker Structures – Tanker Structure Cooperative Forum, Witherby & Co. Ltd., 1995

[4] ISGOTT - International Safety Guide for Oil Tankers & Terminals – ICS & OCIMF- 1986[5] Otegui J., Orsini M.: Converted FPSO’s. Making a Worthwhile Conversion – DOT2004.[6] P. Biasotto, V. Bonniol, P. Cambos : Selection of Trading Tankers for FPSOConversion Projects – OTC2005 [7] Paulo Biasotto & Antoine Rouhan - Survey And Inspection Management forFPSOs - OMAE2004/S&R-51433[8] Rules for Offshore Units – May 2006 – Bureau Veritas.[9] Rules for the Classification of Steel Ships – April 2005 – Bureau Veritas.[10] Floating Production Systems Report, July 2006 - International MaritimeAssociates, Inc. Washington, DC USA

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